Pneumocystis is an opportunistic fungal pathogen which causes severe pneumonia (PCP) in patients with AIDS. During pneumonia, Pneumocystis proliferates through trophic forms and cysts. Mechanisms regulating the Pneumocystis life cycle are not well defined, however a pheromone-induced mitogen-activated protein kinase (MAPK) signaling pathway regulates this process in closely-related fungi. Our studies demonstrate that the P. carinii MAPK PCM, an ortholog to fungal pheromone MAPKs, complements pheromone signaling in yeast. PCM expression and activity are greatly augmented in trophic forms compared to cysts, implicating PCM activity in Pneumocystis life cycle transition. We have discovered that PCM has unique requirements for biochemical kinase activity, and the typical phosphorylation sites required for MAPK function are not needed for PCM kinase activity. These findings suggest novel regulation of PCM, the further study of which might provide insights into signaling pathways hi pathogenic fungi. Additionally, we have identified putative pheromone receptors and a transcription factor as part of this MAPK pathway in P. carinii. In the current proposal, we hypothesize that the pheromone-induced mitogen-activated protein kinase (MAPK) pathway regulates cellular differentiation and proliferation of P. carinii. We will investigate these concepts through three independent but interrelated Specific Aims. Through these investigations we hope to gain insights into P. carinii biology which may provide new information for novel drug development to treat PCP.